Genetic basis of B Locus - corresponds to the TYRP1 gene, which controls eumelanin pigment production responsible for black or brown coat color. The brown coat color is recessive, caused by different variants ("b alleles") in the TYRP1 gene that reduce eumelanin production, shifting pigment from black to various shades of brown. The recessive alleles include commonly known variants like bd, bs, bc, and breed-specific variants such as ba which is reported for the Australian Shepherd type. To express brown coat color, a dog must inherit two copies of recessive brown alleles (e.g., ba/ba or ba combined with other brown alleles). Dogs with one copy of the dominant black allele (B) and one brown allele appear black but carry the brown allele and can pass it on.
Inheritance Pattern - The brown coat color phenotype at TYRP1 is autosomal recessive. Two recessive brown alleles are required to produce the brown coat; otherwise, black pigment predominates. Understanding the specific brown allele type (such as ba in Australian Shepherds) helps breeders track and predict coat color outcomes accurately.
Expression and Breed Specifics - The ba variant is a violet allele valid especially in Australian Shepherds and possibly related breeds. Brown pigmentation affects coat color, nose color, paw pads, and eye rims, which appear brown instead of black. The phenotype can range in shade depending on the exact TYRP1 alleles present and interaction with other coat color genes (such as E and K loci).
Summary - The ba allele at the TYRP1 gene is a recessive brown variant identified in Australian Shepherds contributing to brown coat color. Dogs require two copies of recessive alleles (like ba/ba) to express brown pigment. Genetic testing for ba and other B locus variants allows breeders to predict and control coat color traits effectively in Australian Shepherd breeding programs.